1. Field of the Invention
The present invention relates to a photoelectric conversion device, a photoelectric conversion device material, a photosensor and an imaging device.
2. Description of the Related Art
Conventional photosensors in general are a device fabricated by forming a photodiode (PD) in a semiconductor substrate such as silicon (Si). As for the solid-state imaging device, there is widely used a flat solid-state imaging device where PD are two-dimensionally arrayed in a semiconductor substrate and a signal according to a signal charge generated by photoelectric conversion in each PD is read out through a CCD or CMOS circuit.
The method for realizing a color solid-state imaging device is generally fabrication of a structure where on the light incident surface side of the flat solid-state imaging device, a color filter transmitting only light at a specific wavelength is disposed for color separation. In particular, a single-plate solid-state imaging device in which color filters transmitting blue (B) light, green (G) light and red (R) light, respectively, are regularly disposed on each of two-dimensionally arrayed PD is well known as a system widely used at present in a digital camera and the like.
In this single-plate solid-state imaging device, since the color filter transmits only light at a limited wavelength, light failed in transmitting through the color filter is not utilized and the light utilization efficiency is bad. Also, in recent years, fabrication of a multipixel device is proceeding, and the pixel size and in turn, the area of a photodiode part become small, which brings about problems of reduction in the aperture ratio and reduction in the light collection efficiency.
In order to solve these problems, a system of stacking, in the longitudinal direction, photoelectric conversion parts capable of detecting light at different wavelengths has been proposed. As regards such a system, in so far as visible light is concerned, there are disclosed, for example, a system utilizing wavelength dependency of the absorption coefficient of Si, where a vertical stack structure is formed and colors are separated by the difference in the depth (Patent Document 1), and a system where a first light-receiving part using an organic semiconductor and second and third light-receiving parts each composed of Si are formed (Patent Document 2).
However, such a system is disadvantageous in that the color separation is poor, because the absorption range is overlapped among respective light-receiving parts in the depth direction of Si and the spectroscopic property is bad.
Also, development of a solid-state imaging device having a structure where an organic photoelectric conversion film is formed on a signal read-out substrate is proceeding.
In such a solid-state imaging device, it is a task particularly to enhance the photoelectric conversion efficiency or reduce the dark current, and as a method for improving these properties, there are disclosed, for example, introduction of a pn-junction (Non-Patent Document 1) or introduction of a bulk heterojunction structure (Patent Document 3) for the former and introduction of a blocking layer (Patent Document 4) for the latter.
In the case of increasing the photoelectric conversion efficiency by the introduction of pn-junction or bulk heterojunction structure, an increase in the dark current often becomes a problem. Also, the degree of improvement in the photoelectric conversion efficiency differs depending on the combination of materials and in some cases, the ratio of photo signal amount/dark time noise does not increase from that before introduction of such a structure. In employing the method above, what materials are combined is important and in particular, when reduction in the dark time noise is intended, this is difficult to achieve by conventionally reported combinations of materials.
As the literature describing a photoelectric conversion device using an organic material, Patent Documents 5 to 7, Non-Patent Documents 2 and 3, and the like are also known.
Patent Document 5 describes a device using an organic photoelectric conversion film containing fullerenes, but it is impossible only by fullerenes to satisfy all of the above-described high photoelectric conversion efficiency, low dark current and high light absorption.
Patent Document 6 describes a heterocyclic compound containing thiophene, furan or pyrrole, and Patent Document 7 and Non-Patent Documents 2 and 3 describe a solar cell by an organic photoelectric conversion device using a thiophene derivative and a fullerene derivative.
[Patent Document 1] U.S. Pat. No. 5,965,875
[Patent Document 2] JP-A-2003-332551 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)
[Patent Document 3] JP-A-2002-076391
[Patent Document 4] JP-A-5-129576
[Patent Document 5] JP-A-2007-123707
[Patent Document 6] JP-A-2005-132914
[Patent Document 7] JP-A-2007-091714
[Non-Patent Document 1] Appl. Phys. Lett., 1986, 48, 183
[Non-Patent Document 2] J. Am. Chem. Soc., 2006, 128, 3459
[Non-Patent Document 3] Chem. Commun., 2008, 48, 6489